Among wireless accessing systems, used in radio communication systems, such as portable telephone system, there is a CDMA (Code Division Multiple Access) system.
In the CDMA cellular communication system, it is attempted to increase the channel capacity by splitting an area or cell covered by a radio base station into plural sub-areas or sectors and by changing e.g., the code phase from one sector to another. For constituting a sector, the routine practice is to use plural directive antennas, the cells of each base station are classified depending on the orientation as seen from the base station and are associated with respective sectors, by way of a directivity antenna sector configuration. The antenna directivity and the antenna mounting direction are determined so that the respective sectors will be overlapped to a certain extent. For a mobile station lying in this overlapped area, overlapping sectors are determined. The transmission signals from a mobile station lying in the overlapping areas are despread demodulated in both overlapping sectors and synthesized with weights to demodulate data. Thus, for a mobile station moving during communication across different sectors, momentary interruption caused on sector switching maybe eliminated by soft handoff.
The CDMA system is scheduled to be applied to an IMT-2000 (International Mobile Telecommunication 2000) W-CDMA (Wideband-Code Division Multiple Access) mobile communication system, for which standardization is now going on in ITU (International Telecommunication Union) as the next-generation system.
In the W-CDMA system, the base station transmits the information with the necessary minimum power to a terminal in order to suppress interference to other cells. For realizing this, the terminal side perpetually monitors the quality of the signal received. If the signal quality is lower than the required quality, the base station is commanded to increase the transmission power. If conversely the signal quality is higher than the required quality, the base station is commanded to decrease the transmission power, by way of transmission power control (TPC).
The base station in the W-CDMA system sends information data, time-multiplexed with a known pilot symbol on the reception side, as transmission data.
The mobile terminal extracts the pilot symbol from the received signal and estimates a signal component S and an interference component I from the pilot symbol to calculate an SIR. If the calculated SIR is larger than or smaller than a threshold value, the mobile terminal sets 0 or 1 as TPC data, respectively, and transmits this TPC data as it is time-multiplexed with the information data to the base station.
The base station extracts the TPC data from the received signal and, based on this TPC data, updates the gain of an amplifier for transmission every TPC data. Specifically, the base station raises or lowers the gain by 1 dB if the TPC data is 1 or 0, respectively.
In this manner, the mobile terminal adjusts the transmission power of the base station to a minimum level, with which the reception quality as required is assured, responsive to characteristics of the radio propagation path, which are changing with lapse of time. What is crucial in this case is that a command for raising or lowering the transmission power to the base station, as determined on the side mobile terminal, is transmitted to the base station as quickly as possible and that the TPC bit command from the mobile base station will be reflected on the power on the side base station as quickly as possible.
Meanwhile, in the W-CDMA system, the pilot symbol transmitted by the base station and the TPC bit transmitted by the terminal are afforded with offset, as shown in FIG. 1. This suppresses the delay as from the monitoring of the reception quality to the TPC transmission and to the control of the base station transmission power to one slot. By shortening processing time for the above processing, it is possible to set the base station transmission power responsive to propagation characteristics (reception quality) at the current time point.
Among the techniques of improving the quality of received signals, there is site diversity of simultaneously receiving signals from plural base stations and summing the signals from the respective base stations for demodulation (synthesis). In the radio communication system, employing the CDMA system, this site diversity can be applied readily, and is also used in the W-CDMA system.
A mobile terminal 303 in the W-CDMA system employing the site diversity simultaneously receives signals from a first base station 301 and a second base station 302, controlled by a control station 300, and performs site diversity processing in accordance with a basic sequence shown in FIG. 2.
That is, the mobile terminal measures the signal strength of the neighboring base station during the time the terminal is communicating with the first base station A. At step S1, the mobile terminal detects that the strength of the received signal is increased, and sends a “measurement report message”.
In the “measurement report message”, there are contained an ID and reception signal strength of the first base station 301, the mobile terminal is now communicating with, an ID and the reception signal strength of the newly detected second base station B and the information on the relative reception timing of the first base station A.
At step S2, the control station detects the “measurement report message” to detect that the second base station B has newly been added.
At step S3, the control station commands the second base station B to adjust the transmission timing and to start transmission to the mobile terminal.
At step S4, the second base station B sets the transmission timing to a timing as close to the commanded timing as possible, in accordance with a command from the control station, to re-initiate transmission to the mobile terminal. At step S5, the second base station B sends a post-adjustment transmission timing report to the control station.
Using the transmission timing information of the second base station B, detected at step S6, the control station sends an “Active Set Update” message at step S7 to the mobile terminal. In the “Active Set Update” message, there are contained the IDs and the timing information of the first and second base stations A, B, so that the mobile station is commanded to receive the base station contained in this message.
On detection of the “Active Set Update” message at step S8, the mobile terminal commences to receive the second base station B and returns an “ACK” at step S9 to indicate that the processing has been finished as regularly.
The mobile terminal measures the signal strength from the base stations A, B at all times. The mobile terminal at step S10 detects that the reception signal of the first base station A has become weaker than a receivable signal level and sends a “measurement report message”.
In the “measurement report message”, there are contained the IDs, reception levels and the timing information of the first and second base stations A, B, indicating the low signal reception level values of the first base station A.
If the control station at step S11 detects the “measurement report message” to recognize that the reception level of the first base station A is low, the control station at step S12 sends an “Active Set Update” message. In the “Active Set Update” message, sent at this time, there is contained only the ID of the second base station B.
If the mobile terminal at step S13 receives the “Active Set Update” message, it recognizes only the ID of the second base station B in the message, and accordingly stops receiving the signals from the first base station A. Simultaneously, the mobile terminal at step S14 returns an “ACK” indicating that the processing has been finished as regularly.
On reception of the “ACK”, the first base station stops transmission to the mobile terminal.
The number of the base stations with which the receiving station communicates is updated in this manner at all times.
FIG. 3 shows the reception timing and the transmission timing at the time of site diversity. In this site diversity, there are occasions where the timings of signals received from the respective base stations are not coincident because of different distance from the base station and non-synchroneity between the different base stations. Thus, the mobile station has to synthesize signals received at different timings, such that the mobile station first awaits reception of signals from the base station with the latest reception timing, usually the signal from the base station remotest from the mobile station, before proceeding to synthesize the signals from the respective base stations.
On the other hand, the transmission timing of the mobile terminal is defined by the offset from the signal from the base station with the earliest reception timing, such that the mobile terminal is required to keep this transmission timing rigorously.
Thus, if the reception timing difference among the base stations is increased due to clock frequency shift among the base stations or due to movement of the mobile terminal, the time until fulfilment of synthesis is considerable, as shown in FIG. 4, with the result that the transmission of the TPC bits is not made in time.
There is also raised a problem that, since the transmission of the TCP bit is not made in time, the TPC information as sent ceases to be accurate, with the result that the base station cannot keep optimum transmission power.
There is also presented a problem that, if the TCP bit is determined using only a pilot symbol of the base station A of FIG. 3, the transmission power which is more than is necessary may be demanded of the base station because the signal strength of the base station B is not taken into account in signal summation.